1. Field of the Invention
The present invention relates to a color television system which maintains high definition together with significantly reduced crosstalk between frequency interleaved luminance and chrominance components of a received video signal, and particularly to such a system wherein at both the transmitter and receiver comb filtering is employed to a variable extent corresponding to the variation in vertical correlation of luminance and chrominance on successive scanning lines of the video picture, thereby minimizing crosstalk without loss of high frequency components of the luminance and chrominance signals which provide fine detail in the video picture.
2. Description of the Related Art
Existing standards for broadcast television were originally developed for monochrome transmission. However, in order to maintain compatibility with present and contemplated higher definition color television, as required by national regulatory authorities, the frequency band of the chrominance component of the composite color video signal overlaps the upper portion of the frequency band of the luminance component. This is achieved by frequency interleaving, which is possible if the luminance signal substantially only includes frequencies which are clustered at even harmonics of half the line scanning frequency, so that the frequency components of the chrominance signal can be placed there-between by transmitting the chrominance signal in the form of amplitude modulation of a subcarrier wave at an odd multiple of half the line scanning frequency. In the NTSC system, relative to the frequency of the carrier of the complete video signal the color subcarrier is at about 3.5 MHz, the luminance (Y) signal extends to 4.2 MHz, the sidebands of the Q component of the modulated chrominance signal extend 0.5 MHz to each side of the subcarrier, and the sidebands of the I component of the modulated chrominance signal extend from 1.5 MHz below to 0.6 MHz above the subcarrier. The subcarrier wave itself is suppressed in the transmitted video signal.
Formation of the transmitted video signal from the original RGB signals from a video camera is referred to encoding, and recovery of the RGB signals from the received video signal at the receiver is referred to as decoding. The decoding operation includes separating the modulated chrominance signal from the luminance signal, which has most commonly been effected by bandpass filters. However, that inevitably loses luminance detail since it is comprised in the upper portion of the luminance band which is overlapped by the chrominance band and so will be separated with the chrominance signal. This also results in crosstalk between the luminance and chrominance signals, producing effects in the received video picture known as "cross-color" and "cross-luminance", or more popularly as "rainbow" and "dot" patterns, respectively.
It is therefore preferable to effect decoding of the video signal by comb filtering, employing line, field or frame delays for that purpose. Such filtering presumes, as a result of interleaving of the luminance and chrominance signal frequency components, that the phase of the luminance signal will remain substantially the same on successive scanning lines during successive fields of the video picture, whereas the phase of frequency components of the chrominance signal will alternate on successive scanning lines of successive fields, requiring four fields (1/15 seconds) to repeat. However, such vertical phase relationships between the luminance and chrominance signals are only true in regions of the video picture having a high degree of vertical correlation, i.e. no substantial changes in the vertical direction from line to line. If there are vertically stepped or diagonal shapes in the picture, the frequency components of the luminance and chrominance signals will not be respectively clustered only at even and odd harmonics of half the line scanning frequency, and will overlap to an extent dependent on such departure from vertical correlation in the picture. In such cases, comb filtering would result in loss of high frequency components of the luminance and chrominance signals, with consequent loss of detail in the picture.
Various methods have been proposed to provide comb filtering of the interleaved luminance and chrominance signals while minimizing loss of detail. For example, U.S. Pat. No. 4,688,080, issued Aug. 18, 1987, describes an adaptive chrominance separation circuit wherein a comb filtered chrominance signal is provided for portions of the video picture having high vertical correlation, a lowpass filter otherwise being employed to separate the luminance and chrominance signals. Since luminance detail is above the passband of such a filter, in order to preserve such detail the aforesaid patent teaches to make the decision to either bypass or not bypass the comb filter depend not only on the presence of vertical chrominance detail but also on the presence of luminance detail. However, such adaptation is only an approximation based on certain discrete levels of the changes in the chrominance and luminance signals on successive scanning lines. Such patent also teaches to switch between a combed and a bandpass filtered chrominance signal, and to provide an average of the two during switching from one to the other. However, there is no adaptation of the relative proportions of said signals in accordance with the degree of vertical correlation in the video picture.
Applicant's own published article "Adaptive Filter Techniques for Separation of Luminance and Chrominance in PAL TV Signals", I.E.E.E. Trans. on Consumer Electronics, Vol. CE-32, No. 3, August 1986, describes an adaptive filter for separation of luminance and chrominance which effects a continuously variable transition between line comb filtering and frame comb filtering depending on the degree of vertical change in the picture from frame to frame or from line to line caused, for example, by movement in the picture. Movement adaptive "fading" between frame and line comb filtering is thereby achieved to provide luminance-chrominance separation to an extent consistent with preservation of high frequency detail.
The article "Some Thoughts on Using Comb Filters in the Broadcast Television Transmitter and at the Receiver" by R. Turner, I.E.E.E. Trans. on Consumer Electronics, Vol. CE-23, No. 3, August 1977, describes comb filtering at the receiver as well as at the transmitter so as to exclude frequency components in the transmitted video signal which are not clustered at either even and odd harmonics of half the line frequency. That renders the comb filtering at the receiver more effective in regions of the picture having low vertical correlation, such as diagonals, but excessive exclusion of other frequency components of the video signal will cause noticeable degradation of vertical resolution in the picture. Best results are achieved if the transmitter encoder and receiver decoder employ the same type of comb filtering, such as by line or by frame. However, crosstalk will still occur because with comb filters having a reasonable number of line and frame delay elements there will be some overlap of the luminance and chrominance passband slopes of the filter. Such overlap will also result in loss of vertical resolution.